A Battery Made of Molten Metals

Estimated reading time: 8 minutes

But there was a problem. To keep the components melted, the battery had to operate at 700 degrees Celsius (1,292 degrees Farenheit). Running that hot consumed some of the electrical output of the battery and increased the rate at which secondary components, such as the cell wall, would corrode and degrade. So Sadoway, Bradwell, and their colleagues at MIT continued the search for active materials.

Early results from the magnesium and antimony cell chemistry had clearly demonstrated the viability of the liquid metal battery concept; as a result, the on-campus research effort received more than $11 million from funders including Total and the U.S. Department of Energy’s ARPA–E program. The influx of research dollars enabled Sadoway to grow the research team at MIT to nearly 20 graduate and undergraduate students and postdocs who were ready to take on the challenge.

Within months, the team began to churn out new chemistry options based on various materials with lower melting points. For example, in place of the antimony, they used lead, tin, bismuth, and alloys of similar metals; and in place of the magnesium, they used sodium, lithium, and alloys of magnesium with such metals as calcium. The researchers soon realized that they were not just searching for a new battery chemistry. Instead, they had discovered a new battery “platform” from which a multitude of potentially commercially viable cell technologies with a range of attributes could spawn.

New cell chemistries began to show significant reductions in operating temperature. Cells of sodium and bismuth operated at 560 degrees Celsius. Lithium and bismuth cells operated at 550 C. And a battery with a negative electrode of lithium and a positive electrode of an antimony-lead alloy operated at 450 C.

While working with the last combination, the researchers stumbled on an unexpected electrochemical phenomenon: They found that they could maintain the high cell voltage of their original pure antimony electrode with the new antimony-lead version — even when they made the composition as much as 80 percent lead in order to lower the melting temperature by hundreds of degrees.

“To our pleasant surprise, adding more lead to the antimony didn’t decrease the voltage, and now we understand why,” Sadoway says. “When lithium enters into an alloy of antimony and lead, the lithium preferentially reacts with the antimony because it’s a tighter bond. So when the lithium [from the top electrode] enters the bottom electrode, it ignores the lead and bonds with the antimony.”

That unexpected finding reminded them how little was known in this new field of research — and also suggested new cell chemistries to explore. For example, they recently assembled a proof-of-concept cell using a positive electrode of a lead-bismuth alloy, a negative electrode of sodium metal, and a novel electrolyte of a mixed hydroxide-halide. The cell operated at just 270 C — more than 400 C lower than the initial magnesium-antimony battery while maintaining the same novel cell design of three naturally separating liquid layers.

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